This invention is applicable to all communication systems. A spread-spectrum system will be used to illustrate its use and effectiveness. Spread-spectrum communication systems transmit information digitally on a carrier that has been modulated with a high-rate pseudorandom binary sequence. The spectrum of the resulting signal occupies a large bandwidth and appears noise-like. The signal is subject to intentional or unintentional jamming. Jamming occurs by transmission of large radio frequency signals in nearby or coincident radio spectrum. Jamming signals located out of the desired signal's band are usually removed using a preselector filter. Thus, it is the in band signals that present the greater obstacle to accurate reception of the transmitted signal.
Suppressing these interferences can be accomplished with the use of RF notch filters. When implemented with MEMS technology, this is a low cost, low power solution and low distortion for interference suppression. A notch filter is particularly effective in suppressing continuous wave (CW) or narrowband interferers. After these interferers are removed, the receiver can process the spread spectrum signal as if the interference is absent with a small loss, proportional to the bandwidth being removed. For interference-free spread spectrum signals the received signal can be processed with a very low resolution ADC. For example a 1-bit ADC has a degradation of 1.059 dB with baseband I and Q sampling or 1.96 dB with intermediate frequency (IF) sampling against Additive White Gaussian Noise (AWGN). A 2-bit ADC has a 0.55 dB degradation with baseband I and Q sampling or 0.96 dB with IF sampling. Therefore, to maintain good performance with a low ADC complexity and low power consumption, a method of eliminating the jammers from the received signal should be used.
Another effect on system design and system performance in a jamming environment is in power consumption and signal distortion. In a jamming environment when the jamming signals are not removed near the antenna, the components in the analog front end must be designed with high linearity at the cost of significantly higher power consumption. Also, the presence of large signals puts greater demands on the phase noise requirements of the system. To decrease both the high linearity and phase noise requirements, it is necessary to remove the jammers before these components.
Analog excision methods can eliminate both narrow and wideband jamming signals using passive components. However, since the jamming signals can be located anywhere within the passband, some method for steering, inserting, and removing the excising circuitry from the signal path must be used. Generally, filtering techniques implemented in current receivers use semiconductor switching, e.g., semiconductor transistors, to alter the filters' characteristics. The filters' characteristics may be altered by switching in different components (e.g., banks of capacitors) or different filters altogether. Semiconductor switching, due to the semiconductor's limited isolation characteristics, may allow parasitic capacitances from non-selected filters and/or components to effect the performance of a selected filter, resulting in distortion of the filtered signal.
From the discussion above, it is apparent that there is a need in the art for a low power, low distortion mechanism for protecting a receiver's front end for use in multiple applications.